Recently, there has been an increasing interest in high quality ZnO films due to the electrical and optical properties of ZnO. These properties include wide and direct energy bandgap (Eg˜3.3 eV at room temperature) and large photoresponse, a high free exciton bounding energy, availability of a native substrate, ability to be wet chemical-processed, ability to be grown below the temperature of 400° C., as well as having high resistance to radiation damage. In the past, an optically addressed high contrast UV light modulator was made on ZnO grown on an (01{overscore (1)}2) R-plane sapphire substrate. ZnO films are particularly promising for UV photodetectors due to their wide and direct band gap and large photoresponse. It has been shown that with high quality epitaxial ZnO film, such detectors can achieve high speed operation. Recently, a ZnO p-n diode fabricated using excimer-laser doping has been reported, which makes ZnO promising in other broad device applications, such as Light-Emitting Diodes (LED) and lasers.
In the past, several different metals have been used as a contact metal on ZnO films. For example, epitaxial ZnO film based metal-semiconductor-metal (MSM) device structures have been fabricated using aluminum as the ohmic contact metal (see Ying Liu, C. R. Gorla, S. Liang, N. Emanetoglu, Y. Lu, “J. Electron. Mater.”, 29, 69 (2000)). The device showed fast photoresponse time (˜1 μs) due to high material quality of the ZnO epilayer. However, Schottky type photodetectors are more attractive due to their high speed and low noise performance. An important characteristic of the Schottky diode is its barrier height or turn-on voltage, sometimes called its knee voltage. Schottky barrier heights were first reported by C. A. Mead (see C. A. Mead, “Phys. Lett.”, 18, 218 (1965)), for various metals' contacts on vacuum cleaved n-ZnO surfaces. It has been reported that silver (Ag) has a comparatively high Schottky barrier height (˜0.62 eV) on the cleaved ZnO surface. R. C. Neville and C. A. Mead (see R. C. Neville and C. A. Mead, “J. Appl. Phys.”, 41, 3795 (1970)) further investigated the surface barrier systems consisting of gold and palladium on chemically prepared bulk ZnO. In 1986, H. Fabricius et al. (see H. Fabricius, T. Skeltnep, and P. Bisgaard, “Appl. Opt.” 25, 2764 (1986)), made Schottky barrier type UV photodiodes using gold (Au) as the contact metal on sputtered ZnO thin films. These diodes suffered from severe recombination due to the poor crystalline quality of the film. These photodiodes exhibited slow operating speed and low quantum efficiency due to a large amount of recombination centers in the polycrystalline ZnO layers. Currently, it is believed that no Schottky type of photodetector has been fabricated on ZnO epitaxial films. There is a need for a metal-ZnO Schottky contact which does not suffer from the deficiencies of the prior contacts and which exhibits the advantages mentioned above for broad device applications. It is believed that no Schottky type of photodetector has been fabricated on ZnO epitaxial films prior to this invention.